The hypothesis that cancer results from an impairment of the process of cell differentiation will be tested in tumors of the hematopoetic system. The cell differentiation process may involve the transcription of genes not previously being transcribed. More likely it involves changes in abundancy of specific messengers within the cell. With the use of recombinant DNA techniques to prepare cDNA libraries we can determine the stages of differentiation during which various clones sequences are represented in cytoplasmic mRNA. Upon identification of clones differentially regulated in leukemic cells we propose to determine their chromosome location(s). Thus we will test the hypothesis that cell differentiation involves changes in the transcription frequency of structural genes and relate the appropriate DNA sequences with the known chromosomal abnormalities associated with the disease.